My father (a photographer of 30yrs) taught me to shoot and I always knew he was right handed but shot left. I asked him why and he told me it was because he was left eye dominant; but a righty. I'd never noticed when he took pictures because he was facing me but he also uses his left eye to look through the camera. Dominant eye makes the decision. You can teach him to shoot left handed but his dominant will never change. If he insists on continuing to shoot right handed get him an eye patch. Your cheapest solution.
Best of luck!
Dan